It is often desirable to be able to obtain information directed to the quality or physical characteristics of a solution. In one application, it is worthwhile to obtain water quality data for surface and/or ground water environments. Such data might include hydrogen potential (pH), specific ion potential (pIon) and oxygen reduction potential (ORP), and other information of that nature. In connection with obtaining such data, potentiometric sensors can be used to measure the data. Potentiometric sensors are a class of sensors with characteristically high source impedances (in the range of about 10.sup.8 ohms) and which measure galvanic potential based on the chemical activity in the solution.
In connection with developing a measurement signal representative of the physical parameter using the potentiometric sensor, interface amplifiers are available for amplifying the measurement signal and which have input impedances of 10.sup.13 ohms or greater and bias currents of 10.sup.-12 amperes or less. A high input impedance is required for the interface amplifier because of the high source impedance of the potentiometric sensor. It is also important that bias currents be minimal to avoid polarizing the solution in the vicinity of the potentiometric sensor. Polarization occurs when a direct current is applied to the solution at a rate greater than the ion migration rate within the solution. Polarization causes a cloud of ions of opposite polarity to form about the potentiometric sensor. Because of such ion formation, the measurement signal would have a value different from the true or actual value of the physical parameter of the solution being measured.
Typically, potentiometric measurements are made in laboratory environments in which the measuring instrument or equipment power is applied prior to taking measurements and a "warm-up" time of 5-10 minutes does not detrimentally impact system performance. Such a warm-up period would result in the stabilization of the potentiometric sensor wherein any polarization thereof, which would affect the accuracy of the measurement signal, would be dissipated and no longer present. Recently, however, newer developments of instrumentation involving potentiometric sensors have evolved where such instruments are battery powered. Such instruments include potentiometric sensors that are located in the field or in an environment in which the sensor or equipment need only be powered on for a short time interval within which to obtain the measurement and then is powered down for the remaining time. In such a case, it is detrimental to system performance, particularly battery life, to require or utilize a warm-up period of 5-10 minutes. Such a warm-up period would result in an unacceptable drain on the battery energy and no accurate and useful measurement would be obtainable during this warm-up period.